Evidence of nighttime production of organic nitrates during SEAC4RS, FRAPPE, and KORUS-AQ

Organic nitrates (RONO) are an important NO sink.  In rural environments dominated by biogenic emissions, nocturnal NO-initiated production of RONO is competitive with daytime OH-initiated RONO production.  However, in urban areas, OH-initiated production of RONO has been assumed dominant and NO-initiated production considered negligible.  We show evidence for nighttime RONO production similar in magnitude to daytime production during three aircraft campaigns in chemically-distinct environments: SEACRS in the rural Southeastern US, FRAPPE in the Colorado Front Range, and KORUS-AQ around the megacity of Seoul.  During each campaign, morning observations show RONO enhancements at constant, near-background Ox (≡ O + NO), indicating that the RONO are from a non-photochemical source, whereas afternoon observations show a strong correlation between RONO and O resulting from photochemical production.  We show there are sufficient precursors for nighttime RONO formation during all three c...